The present invention relates generally to parametric frequency divider structures and more particularly to analog frequency dividers that can be implemented as micro-electro-mechanical systems (MEMS).
Frequency multiplication, division, and mixing are signal processing techniques which are utilized in a wide array of devices including electronic, optical, and opto-electronic devices. MEMS structures are emerging as an important technology for manufacturing devices that perform signal processing. For example, U.S. Pat. No. 6,262,464 B1 describes a MEMS device that can be utilized for signal mixing and filtering.
Devices for performing frequency division, e.g., high-speed GaAs digital frequency dividers, are also known. Such conventional digital frequency dividers however, typically require high power for operation, and hence are not particularly suitable for a variety of applications such as chip-scale atomic clocks, miniature radio frequency (RF) receivers and transceivers, and other battery-operated, portable wireless devices.
Accordingly, there is a need for frequency dividers that can operate on low power. Further, there is a need for such frequency dividers that are sufficiently small to be suitable for incorporation in small-scale devices, especially portable communication and navigation devices incorporating RF synthesis and time references.
The present invention provides an analog mechanical parametric frequency divider structure that functions as an injection-locked parametric resonator which can be driven by an input signal at a selected frequency in a manner that a parameter of the resonator is varied so as to generate a coherent oscillation at a fraction of the input frequency as an output signal. For example, the resonator parameter that is varied can be the length of a micro-mechanical oscillatory beam such that flexural motion of the beam occurs at a sub-multiple of the driving input frequency.
In one embodiment, a parametric frequency divider structure according to the invention includes a vibratory beam with a longitudinal axis that extends between two ends, at least one of which is fixed. The divider structure further includes a piezoelectric transducer that is mechanically coupled to the vibratory beam and periodically applies a longitudinal deformation force, i.e. a longitudinal expansion or compression force to the beam. This periodic longitudinal deformation force induces a periodic vibration in the beam in a direction transverse to the longitudinal axis (a bowing or swaging of the beam) at a frequency that is substantially equal to an even sub-multiple, e.g. one-half, of the longitudinal frequency.
In a related aspect, the vibratory beam exhibits a natural vibrational resonance in the transverse direction at a frequency that is substantially equal to an even sub-multiple of the frequency of the longitudinal deformation force. This facilitates inducing a transverse vibration in the beam at the transverse resonance frequency, an even sub-harmonic of the frequency of the longitudinal deformation force. Because of the frequency-dividing properties of the present invention, such structures are particularly useful in electronic signal processing.
For example, in one embodiment, an analog frequency divider is disclosed based on a structure according to the invention, which further includes an electrically conductive layer disposed on at least a portion of the vibratory beam and a conductive electrode that is positioned in proximity of the conductive layer. The electrode couples capacitively to the periodic vibration of the beam to generate an oscillatory electrical signal at the transverse vibrational frequency.
In a further aspect, the vibratory beam can include two surfaces, which extend along the transverse direction on opposite sides of the beam (e.g., on the top and the bottom of the beam), and the piezoelectric transducer can be implemented as a piezoelectrically active film that covers at least a portion of one of the opposing beam surfaces. An oscillator generating a periodic voltage at a selected frequency can be coupled to the piezoelectric film to cause a change in the length and/or effect a longitudinal deformation of the piezoelectric film. The deformation of the film, as a result of the mechanical coupling of the film to the vibratory beam, in turn induces longitudinal deformation of the beam.
An analog frequency divider structure according to the teachings of the invention can be implemented as a micro-electro-mechanical systems (MEMS) device formed, for example, in an integrated circuit chip or wafer. In a MEMS frequency divider of the invention, the vibratory beam can be formed of an insulating material, such as Si3N4. Further, the conductive layer, which covers at least a portion of the beam, can be formed of Si2Co. In such a MEMS device, the conductive electrode, which couples capacitively to the beam, can be formed from a metal, such as, platinum, tungsten, gold or copper.
In further aspects, the invention provides a MEMS cascade chain of analog frequency divider structures, as described above, that are coupled to one another in a series arrangement. The first member of the chain receives an input signal at a selected frequency and the last member of the chain generates an output signal at a fraction of the input frequency. The output of each member, other than the last member, provides an input signal for the next member in the chain. Each member exhibits a transverse vibrational resonance at a frequency that is a fraction of the corresponding resonance frequency of a previous member in the chain. Thus, each member divides the output frequency of a previous neighboring member by a selected fraction to generate the final output frequency. For example, in one embodiment, each frequency divider in the chain divides the output frequency of a previous divider by 2. The members of the chain can be directly coupled to one another, or alternatively, one or more gain stages may be employed between the chain members.
The analog frequency dividers of the invention can find a wide range of applications. Such applications can include, but are not limited to, radio systems (e.g., wireless communication devices), and miniature atomic clocks. For example, an atomic clock system according to the teachings of the invention can include an atomic vapor, such as cesium and a microwave oscillator that generates radiation selected to be in a range suitable for exciting an atomic transition. The atomic clock system further includes a feedback system for monitoring the response of the atoms to the radiation. The feedback system is coupled to the oscillator and applies a feedback signal thereto based on the monitored response, in order to stabilize the oscillator frequency. One or more MEMS analog dividers, such as the divider described herein, can receive a portion of the oscillator radiation at the stabilized frequency as an input signal and generate an output signal at a fraction of the oscillator frequency. This output signal of the MEMS divider (or cascading series of dividers) is preferably in a range, e.g., a few megahertz, that is useful as a clock frequency having a stability and an accuracy commensurate with those of the atomic transition.
Another example of an application for a MEMS analog frequency divider according to the invention is in a frequency synthesizer, such as those utilized in a radio receivers or transceivers, where a low-power frequency divider is needed to prescale a local oscillator to a lower frequency for further processing by digital logic.
The following embodiments, described with reference to the following drawings, provide further understanding of the invention.